1. Field of the Invention
This invention relates to a system and a method for changing the angular momentum of a spacecraft by an automatic magnetic torquing system and more particularly to changing the spin rate of a tumbling orbiting satellite by a magnetic torque to restabilize the satellite.
2. Description of the Prior Art
In the operation of a stabilized orbiting satellite there is always a finite probability that an accident will occur which results in uncontrolled tumbling of the spacecraft. Such uncontrolled motion of a satellite, intended to be stabilized in orbit, renders the satellite effectively useless for its planned operation. Furthermore, prolonged tumbling at excessive angular momentum may jeopardize the structural integrity of the spacecraft as well as the spacecraft's thermal and power balance. Therefore, it is desirable that recovery of the satellite occur within a short period of time after tumbling begins.
"Detumbling" can be accomplished by mass expulsion devices such as, rocket thrusters or jets. The use of such devices to "despin" the tumbling spacecraft often requires excessive propellent usage. In the situation where no propellent is available, such means can not be used to restabilize the spacecraft. Where electrical power in the satellite is available, it is more desirable to detumble the spacecraft by changing the spacecraft's spin rate by an autonomous control system.
Magnetic torquing of stabilized satellites, in particular for changing the attitude of a spacecraft which has deviated from its desired orientation relative to its orbit, is known. Such magnetic torquing systems use a magnetic field from torquers such as coils or electromagnets to interact with the magnetic field of the earth to develop a reaction torque. This reaction torque causes the reference axis of the satellite to be reoriented an amount proportional to the torquing time and flux magnitude as is well known in the art. Magnetic torquing can also be used to develop a reaction torque to control the spin rate and the angular momentum of a spacecraft. These known magnetic torquing systems may be implemented in a satellite or spacecraft with an automatic (closed-loop) control system or an open-loop control system requiring command signals from a ground-situated station.
The problem with such open-loop control systems is that the ground station link in the control system necessitates delays in the correction operations by personnel who must interpret satellite signals and provide the appropriate and timely command signal. Such delays make it difficult to achieve corrections that are best made more frequently as the satellite becomes properly oriented in its earth orbit. The expense of operating ground stations with personnel serving to solve satellite attitude and momentum problems is also a burdensome disadvantage in such open-loop control systems.
Up to the present, most efforts for the automatic recovery of a tumbling satellite focused on the known magnetically torqued despin schemes which utilize suitably integrated and transformed gyro outputs, updated with data from earth sensors to provide a continuous spin phase reference for commutating magnetic coils disposed on the spacecraft or magnetometers utilized to sense the magnitude and direction of the earth's magnetic field to control coil commutation. Although feasible, such techniques are relatively complex and time consuming to implement and require considerable computer software and hardware capability.
Accordingly, a system for automatically recovering a tumbling spacecraft is needed to overcome the deficiencies of the presently known proposals and systems. Such an autonomous system, utilizing on-board hardware to produce a magnetic torque to "despin" the spacecraft, i.e., reduce the spin rate of the spacecraft along the spin axis, particularly when despinning cannot be accomplished by mass expulsion devices such as rocket thrusters, is desirable.